This invention relates to a measurement method or an imaging position detecting method for detecting a best focus position, a best image plane or an astigmatism aberration of an optical system, for example. The present invention is particularly suitably usable for detecting an optical characteristic of a projection optical system in a projection exposure apparatus used in a lithographic process for the manufacture of semiconductor devices, liquid crystal display devices or thin film magnetic heads, for example.
The manufacture of semiconductor devices, liquid crystal display devices, or thin film magnetic heads, for example, based on a lithographic process uses a projection exposure apparatus for forming an image of a pattern of a photomask or reticle (hereinafter, simply xe2x80x9creticlexe2x80x9d) on a photosensitive substrate through a projection optical system. In such a projection exposure apparatus, in order that the reticle pattern is printed on the photosensitive substrate with a high resolution, the exposure process should be carried out while the photosensitive substrate is registered with the best imaging plane (best focus plane) of the projection optical system, within the tolerance of the depth of focus. To this end, by using any method, the best focus plane of the projection optical system, that is, the best focus position thereof has to be detected. However, in practical projection optical systems, there are various aberrations involved. Particularly, if there is astigmatism, the best focus position then differs with the direction of advancement of light diffracted by the pattern. Further, a projection optical system has a best focus image plane in which the best focus position differs with the image height.
In consideration of the above, in conventional measuring methods for measuring the best focus position or astigmatism of a projection optical system, exposures are made while successively changing the position of a photosensitive substrate with respect to the optical axis direction of the projection optical system and CD changes of resist images formed on the surface of the photosensitive substrate at different positions are measured. Alternatively, changes of contrast of an aerial image of a pattern formed by the projection optical system at different image heights are measured to determine the best focus position, to thereby measure the image plane. As a further alternative, the difference in best focus position of a pattern with respect to two different directions of sagittal direction and meridional direction is detected
There is a further method in which a line pattern is formed on a reticle by use of chromium (light blocking material), with glass portions (light transmitting portion) defined on the opposite sides of the line pattern. A pattern having different phases of 90 deg. and 0 deg. is used in the light transmitting portion, such that, on the basis of a positional deviation resulting from distortion of imaging due to defocus, the best focus as well as a difference in best focus between two orthogonal directions is detected, from the relation between the positional deviation and the focus. The image plane is measured in this manner.
These conventional measurement methods involve difficulties in quantization of the measurement results and, thus, difficulties in performing high precision measurement.
For simplification of the quantization, there is a method for detecting the best focus position in accordance with the relation between the focus and the spacing of projected images formed at different positions of the same pattern or different patterns, being illuminated at different chief ray incidence angles and being projected.
Even with such a quantization-simplified method, there are difficulties in accomplishing high precision measurement. This is because, in such a method, projected pattern images must be formed at different positions and, therefore, as the spacing between the projected pattern images becomes large, the detection error necessarily becomes large. In order to make the image spacing smaller, the projected pattern image itself, that is, the original pattern itself, has to be made smaller. The work precision is then insufficient.
On the other hand, this method is limited to the best focus position measurement. It is not applied to quantization for the image plane measurement or astigmatism measurement.
Further, in the measurement, not only the mask, but also the illumination system has to be manipulated specifically.
It is accordingly an object of the present invention to provide a measurement method and/or a projection exposure apparatus using the same, by which focus information can be measured conveniently and very precisely.
It is another object of the present invention to provide a measurement method and/or a projection exposure apparatus using the same, by which convenient and high precision quantization can be accomplished in the measurement of an image plane.
It is a further object of the present invention to provide a mask which is suitably usable with the measurement method such as described above and by which the necessity of special manipulation of an illumination system is removed in principle.
It is a yet further object of the present invention to provide a measurement method by which astigmatism at different image heights of a projection optical system can be detected quickly and very precisely.
It is a still further object of the present invention to provide a measurement method and an exposure apparatus using the same, by which an imaging position characteristic of a projection optical system, for example, can be measured very precisely.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.